In recent years, from the viewpoint of soaring oil prices and exhaustion of oil resources estimated in the near future, deep oil wells which have not been searched, oil wells and gas wells in severe corrosive environments under so-called sour environments have been actively developed. Oil country tubular goods use in such environments are required to include materials having both high strength and excellent corrosion resistance (sour resistance).
For this requirement, for example, Patent Literature 1 describes steel for oil country tubular goods having excellent resistance to sulfide stress cracking (SSC-resistance), the steel containing C: 0.15 to 0.35%, Si: 0.1 to 1.5%, Mn: 0.1 to 2.5%, P: 0.025% or less, S: 0.004% or less, sol. Al: 0.001 to 0.1%, Ca: 0.0005 to 0.005%, and a Ca-based non-metallic inclusion having a composition containing CaS and CaO in a total of 50% by mass or more and a Ca—Al compound oxide at less than 50% by mass, and the steel having hardness in a range of 21 to 30 HRC and a specified relation between the hardness and the total amount X (% by mass) of CaO and CaS. A technique described in Patent Literature 1 includes accelerating a reaction with harmless CaS and CaO by decreasing the amount of the Ca—Al compound oxide adverse to SSC-resistance, producing steel for oil well use having improved SSC-resistance.
Patent Literature 2 describes a method for producing a seamless steel pipe having small variation in strength and a microstructure with austenite grain size No. 6 or more according to ASTM standards, the method including piercing and hot-rolling a billet, forming a pipe under the condition of a finish rolling temperature of 900° C. to 1100° C. to produce a seamless steel pipe, and quenching the steel pipe while maintaining it in a temperature region equal to or higher than an Ar3 point and tempering, the billet having a composition containing C: 0.15 to 0.35%, Si: 0.1 to 1.5%, Mn: 0.1 to 2.5%, P: 0.03% or less, S: 0.005% or less, sol. Al: 0.001 to 0.1% or less, Cr: 0.1 to 1.5%, Mo: 0 to 1.0%, N: 0.0070% or less, V: 0 to 0.15%, B: 0 to 0.0030%, Ti: 0 to A % wherein A=3.4×N (%), and Nb: 0.005 to 0.012%. A technique described in Patent Literature 2 includes forming a microstructure by adjusting the steel composition and the finish rolling temperature, thereby decreasing strength variation.
In addition, Patent Literature 3 describes a method for producing a seamless steel pipe having high strength and high corrosion resistance. A technique described in Patent Literature 3 relates to a method for producing a seamless steel pipe by quenching and tempering a steel pipe and then applying plastic strain with a sectional plasticity rate of 10 to 90% to the steel pipe in a hot manner at 400° C. to 750° C., the steel pipe containing C: 0.30% or less, Si: 0.05 to 1.00%, Mn: 0.30 to 1.20%, S: 0.03% or less, Cr: 0.50 to 1.50%, Mo: 0.10 to 2.00%, Ni: 0.50% or less, and Cu: 0.10% or less. The technique described in Patent Literature 3 decreases the hardness of inner and outer surface layers of the steel pipe, which come into contact with a corrosive atmosphere, producing a seamless steel pipe satisfying both high strength and high corrosion resistance.
Patent Literature 4 describes steel having excellent resistance to sulfide cracking. A technique described in Patent Literature 4 includes controlling a composition to contain C: 0.01 to 0.10%, Si: 0.05 to 0.60%, Mn: 0.50 to 2.50%, P: 0.010% or less, S: less than 0.002%, Al: 0.005 to 0.100%, Ti: 0.005 to 0.020%, and Ca: 0.0005 to 0.0050%, and controlling micro Vickers hardness to 250 or less and a deviation in hardness in the thickness direction to 60 or less, improving the resistance to sulfide cracking of steel.
Patent Literature 5 describes a method for producing a high-strength corrosion-resistant steel pipe. A technique described in Patent Literature 5 includes quenching and tempering a steel pipe two times, the steel pipe containing C: 0.30% or less, Si: 0.05 to 1.00%, Mn: 0.30 to 1.00%, P: 0.03% or less, S: 0.03% or less, Cr: 0.30 to 1.50%, Mo: 0.10 to 2.00%, Al: 0.01 to 0.05%, and N: 0.015% or less, and further containing at least one of Nb: 0.01 to 0.04%, V: 0.03 to 0.10%, Ti: 0.01 to 0.05%, B: 0.0010 to 0.0050%, and Ca: 0.0010 to 0.0050%, wherein complete bend removal is performed in a cold or hot manner after first quenching and tempering, and slight or no bend removal is performed after second quenching and tempering, thereby producing a high-strength corrosion-resistant steel pipe having a small bend and a uniform hardness distribution of 110 ksi grade or more in the wall thickness direction.
[PTL 1] Japanese Unexamined Patent Application Publication No. 2002-60893
[PTL 2] Japanese Unexamined Patent Application Publication No. 2000-219914
[PTL 3] Japanese Unexamined Patent Application Publication No. 05-287380
[PTL 4] Japanese Unexamined Patent Application Publication No. 07-166293
[PTL 5] Japanese Unexamined Patent Application Publication No. 05-287381